 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a position detector utilizing a magnetostrictive
effect, and more particularly to a position detector for detecting the
position of an elongated movable object which provides high detection
accuracy even in the atmosphere in which a temperature change occurs.
In a prior position detector, a core wire is disposed inside a waveguide
made of a ferromagnetic material and the movable object is provided with a
permanent magnet at a predetermined position thereof. When a pulse current
is applied from a pulse generator to the core wire, ultrasonic wave is
generated by a magnetostrictive effect at the location where the permanent
magnet of the movable object exists, and propagates through the waveguide.
If the permanent magnet exists at a position having a distance x from some
point on the waveguide, a voltage can be detected in a coil at the time t
(sec) after the application of the pulse current by the inverse
magnetostrictive effect. The position x can be determined in accordance
with the formula below by measuring this time t:
x=v.multidot.t
where v is the propagation speed of the ultrasonic wave inside the
waveguide.
In conventional position detectors of the kind described above, the
propagation time t of the ultrasonic wave is measured in accordance with
the formula above. Therefore, if the propagation speed v changes for some
reason or other, the time t also changes. Thus, the change of v directly
turns out to be the detection error.
SUMMARY OF THE INVENTION
The present invention is therefore directed to provide a high precision
position detector having a limited detection error due to a temperature
change.
The present invention is characterized in that a plurality of permanent
magnets are disposed on a movable object with predetermined spacings
between them, fixed permanent magnets are disposed at positions spaced
apart by a predetermined distance from the detecting portion of a
waveguide, the magnetostrictive ultrasonic waves occurring in the
waveguide are detected at the positions of these magnets, and the position
of the movable object is detected from both the number of ultrasonic
signals and propagation time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a position detector of the prior art;
FIG. 2 shows the principle of the operation;
FIG. 3 shows the temperature dependence of the propagation speed of
ultrasonic wave;
FIG. 4 shows the construction of the present invention;
FIG. 5 shows a circuit diagram of one embodiment of the invention;
FIG. 6 shows the principle of operation of the circuit;
FIGS. 7 through 9 show examples of detected pulses depending upon magnet
positions;
FIG. 10 shows a driving mechanism of a reactor control rod; and
FIG. 11 shows an application example of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring initially to FIGS. 1 through 3, the prior art technique described
earlier will be explained in detail.
In FIG. 1, a core wire 2 is shown disposed inside a waveguide 1 made of a
ferromagnetic material such as a Ni-Fe alloy or the like. When a pulse
current is applied from a pulse generator 4 to the core wire 2, an
ultrasonic wave is generated in the waveguide 1 by the magnetostrictive
effect at a position where permanent magnet 5 disposed on a movable object
exists. A protective pipe 3 is disposed lest the waveguide comes into
contact with external objects and the ultrasonic wave is damped. The
resulting ultrasonic wave reaches a guide 6 which contacts the waveguide
and serves for guiding the propagated ultrasonic wave to a detecting
position as described hereafter. A coil 8 is wound on the guide 6, and a
permanent magnet 7 is disposed in the proximity of the coil 8. Therefore,
when the ultrasonic wave arrives at the guide 6 passing through the inside
of the coil 8 which exists in the magnetic field generated by the
permanent magnet 7, a voltage is induced in the coil 8 by the inverse
magnetostrictive effect, so that the generated and propagated ultrasonic
wave is detected.
If the permanent magnet 5 exists at a position having a distance x from the
guide 6, the voltage can be detected in the coil 8 at the time t (sec)
after the application of the pulse current, as shown in FIG. 2(2). The
position x can be determined by the following formula by measuring this
time t:
x=v.multidot.t (1)
where v is the propagation speed of the ultrasonic wave inside the
waveguide 1.
More particularly, a flip-flop 10 is set simultaneously with the
application of the pulse current as shown in FIG. 2(1), and is reset when
the coil 8 detects the ultrasonic wave. For this reason, the output of the
flip-flop 10 becomes "1" only for the time t which is proportional to the
position x as shown in FIG. 2(3). A d.c. output proportional to x appears
in an indicator 13 after the output is smoothed by a resistor 11 and a
capacitor 12.
In the conventional position detector of this kind, the propagation time t
of the ultrasonic wave in the formula (1) is measured, so that t will
change if the propagation speed v changes for some reason or other, and
the change of v results directly in an error in the detection of x.
FIG. 3 shows the temperature dependence of the propagation speed of the
ultrasonic wave inside a nickel steel. It has a temperature dependence of
approximately 1.5.times.10.sup.-2 %/.degree.C., and a detection error of
about 4.5% appears if the ambient temperature changes by 300.degree. C.
When this position detector is used as a position detector for detecting
the control rod of a nuclear reactor, for example, there is the
possibility that a detection error of about 83 mm occurs.
Hereinafter, one embodiment of the present invention will be described in
detail with reference to FIGS. 4-6.
FIG. 4 illustrates a position detector of the present invention. In the
drawing, like reference numerals are used to identify like constituents as
in FIG. 1. FIG. 5 shows a circuit diagram of the position detector of the
present invention.
The differences of the position detector shown in FIG. 4 from the prior art
detector shown in FIG. 1 are that a plurality of permanent magnets 5
(hereinafter called "movable permanent magnets 5") are disposed on the
movable object 5' with a predetermined spacing l between adjacent
permanent magnets 5. Further, the magnet 5 at the left end of the movable
object 5' is disposed at a position spaced apart by a distance .alpha.
from a reference point of the movable object which in a reference position
is aligned with the guide 6. Additionally, two permanent magnets 14a and
14b (hereinafter called "fixed permanent magnets 14") are disposed at
positions spaced apart by different distances .beta. and .gamma. from the
right end of the waveguide 1 opposite to the guide 6.
When a pulse current such as shown in FIG. 6(1) is applied from the pulse
generator 4 to the core wire 2, magnetostrictive ultrasonic waves are
generated at respective positions of the waveguide 1, at which the movable
permanent magnets 5 and the fixed permanent magnets 14a and 14b apply
magnetic fields to the waveguide. These ultrasonic waves propagate through
the waveguide 1, reach the guide 6 and induce voltages in the coil 8. The
voltages are then subjected to wave shaping and are amplified, providing
pulse signals as shown in FIG. 6(2) from a terminal 51.
A signal is applied from a terminal 50 to a latch 55 shown in FIG. 5
simultaneously with the application of the pulse current to the core wire
2, and the content of a counter 54 is latched by the latch 55. The counter
54 is reset by a pulse passing through a delay circuit 53, as shown in
FIG. 6(6), which sets a delay time .tau..sub.53 that is longer than the
time required for the latch operation. If this delay circuit 53 does not
exist, the latching operation of the content of the counter 54 by the
latch 55 starts simultaneously with the resetting operation of the counter
54, so that the latching operation of the content of the counter 54 into
the latch 55 can not be carried out normally.
The detected pulses appearing at terminal 51 as shown in FIG. 6(2) trigger
a retriggerable mono-multi vibrator 56 (hereinafter called "R.M.M.") and
this output Q becomes such as shown in FIG. 6(4). Here, the set time
.tau..sub.R of R.M.M. is smaller than the propagation time required for
the ultrasonic wave to transfer between the adjacent movable permanent
magnets. Since R.M.M. 56 operates at the leading edge of the detected
pulse shown in FIG. 6(2), a flip-flop 59 would be reset simultaneously
with the application of the first detected pulse to the terminal 51.
However, a delay circuit 57 having a delay time .tau..sub.57 as shown in
FIG. 6(5) which is greater than the time width of the detected pulse is
disposed at the output of R.M.M. 56 to prevent such simultaneous
resetting. The circuit shown in FIG. 5 is directed to count the detected
pulses appearing at terminal 51 until the period or interval between the
detected pulses becomes different as shown in FIG. 6(2), and the flip-flop
59 is reset at the timing at which the pulses having a different period is
detected as shown in FIG. 6(8). The different period pulses are caused by
pulses corresponding to ultrasonic waves generated by the fixed magnets
14a and 14b. Therefore, the pulse is applied to an AND gate 58 through the
delay circuit 57. In other words, one of the inputs to the AND gate 58
becomes such as shown in FIG. 6(5). The output signal of the AND gate 58
becomes such as shown in FIG. 6(7), and the flip-flop 59 is reset at the
first pulse.
The counter 54 counts the detected pulses that are applied to an AND gate
60 through the delay circuit 52 during the period in which the flip-flop
59 is kept set. Here, the delay circuit 52 is disposed to delay the
detected pulses by only a limited time .tau..sub.52 so as not to count the
pulse that resets the flip-flop 59.
On the other hand, a counter 64 is reset by the pulse current appearing at
terminal 50 or the detected pulses appearing at terminal 51 as shown in
FIG. 6(10). An oscillator 63 is connected to the counter 64 and always
effects counting as shown in FIG. 6(11). A change-over device 65
sequentially produces signals to the latches 66, 67, and 68 by means of
the output signal of the AND gate 58 (FIG. 6(12), and the content of the
counter is sequentially stored in the latches 66, 67 and 68. In other
words, the number of pulses n.sub.1, n.sub.2 and n.sub.3 corresponding to
the pulse intervals t.sub.1, t.sub.2 and t.sub.3 are stored in the
latches.
Thereafter, a calculation circuit 69 performs the following processing.
First, n.sub.1, n.sub.2 and n.sub.3 are added, and the pulse number N
corresponding to the pulse interval T is determined. Next, a division
n.sub.1 /N is effected to determine the relation of position between the
fixed permanent magnets 14 and the movable permanent magnets 5. Next,
another calculation circuit 70 performs the following calculation using
the result of the calculation circuit 69, the content of the latch 55 and
the set conditions l, .alpha., .beta. of the magnets:
##EQU1##
where L is the length of the waveguide, between the guide 6 and the right
end of the waveguide and i is the content of the latch 55.
This value x represents the moving distance of a movable object 5' with the
guide 6 shown in FIG. 4 being the reference point.
The above explains the operation when the fixed permanent magnets 14a and
14b are disposed between adjacent movable magnets 5. However, when a
movable permanent magnet 5 is interposed between the fixed permanent
magnets, 14a and 14b the ultrasonic wave is detected as shown in FIG.
7(2). That is, the ultrasonic wave 41 generated by a movable permanent
magnet 5 in response to a pulse current as shown in FIG. 7(1) is detected
between the ultrasonic waves 42 and 43 generated by the fixed permanent
magnets 14a and 14b. Here, the contents n.sub.1, n.sub.2 and n.sub.3 taken
into the latches 66, 67 and 68 are values that correspond to the time
t.sub.1, t.sub.2 and t.sub.3 shown in FIG. 7(2). The value t.sub.1 in FIG.
7(2) is reliably greater than the value t.sub.1 shown in FIG. 6(10). When
the content n.sub.1 of the latch 66 corresponding to t.sub.1 exceeds a
predetermined value m as described below, the change-over device 65 is
prevented from producing the latch signal to the latch 68, and the latch
68 is cleared. Therefore, the content n.sub.3 of the latch 68 in this case
becomes zero, and thereafter the position is detected in the same way as
described above. Here, the value m is set to a value n.sub.1 which is
taken into the latch 66 when the position of the fixed permanent magnet
14b close to the end of the waveguide overlaps with the position of one of
the movable permanent magnets 5. Therefore, no problem occurs even when
the fixed permanent magnet 14b overlaps with the movable permanent magnet.
As the third case, there is a case where the fixed permanent magnet 14a
overlaps with one of the movable permanent magnets 5. The detected pulses
in this case are shown in FIG. 8(2) because the ultrasonic wave by the
fixed permanent magnet 14a overlaps with the ultrasonic wave by one of
the movable permanent magnets 5 and is represented by a single detected
pulse 44. Therefore, the values n.sub.1 and n.sub.2 corresponding to
t.sub.1 and t.sub.2 shown in FIG. 8(2) are taken into the latches 66 and
67, but n.sub.3 is zero because the latch 68 does not operate. The t.sub.1
value in this case is equal to t.sub.2 shown in FIG. 6(10). Therefore, if
the magnets are disposed in such a manner as to satisfy the relation
t.sub.1 <t.sub.2, the relation of the position of the magnets can be
discriminated by checking the condition t.sub.1 <t.sub.2. In this
instance, the position can be detected by reducing the spacing
(.beta.-.gamma.) between the fixed permanent magnets from the value
obtained by equation (2).
The description given above deals with the case where two fixed permanent
magnets are disposed, but the number of the fixed permanent magnets may be
one. The detected signal in this case becomes such as shown in FIG. 8(2),
and only t.sub.1 and t.sub.2 are measured whereby the latch 68 shown in
FIG. 5 becomes unnecessary. If a device for monitoring the data on the
measured position is disposed in order to cope with the overlap of the
magnets, the position detector becomes possible.
Furthermore, the movable permanent magnets may be disposed in such a manner
as to satisfy the following relation between the period T.sub.p of the
pulse current and the ultrasonic pulse interval T by the movable permanent
magnets:
T.sub.p =nT+T.sub..alpha. (3)
where n is the number of the movable permanent magnets, and T.sub..alpha.
is the time required for the ultrasonic wave to propagate over the
distance .alpha. as shown in FIG. 4.
In this case, a signal such as shown in FIG. 9(2) is detected when the
movable object moves. The value t changes between O and T. The position
can be detected by merely measuring the time t if the pulse interval T is
set before the time t as shown in FIG. 9(2) or is known in advance from
the set spacing between the movable permanent magnets.
An example of application of the position detector in accordance with the
present invention to a position detector for the control rod of a nuclear
reactor will be explained with reference to FIGS. 10 and 11. The control
rod is connected to a hollow piston 105, and is operated by the rotation
of a ball screw 104 when the system operates. It is disengaged from the
latch 106 at the time of scrum, and is operated by the pressure of the
water entering from a scram piping arrangement 107. FIG. 11 shows the
fitting position of the position detector of the present invention, where
it is shown disposed between an outer tube 102 and a guide tube 103. The
fixed permanent magnet 14 is fixed to the outer tube, while the movable
permanent magnets 5 are fitted to the hollow piston 105, so that the
position of the hollow piston 105, that is, the position of the control
rod, can be detected by the operation described above.
The detection error when the full stroke of the control rod is 3,660 mm and
the number of the movable permanent magnets is 10, is about 1.6 mm
provided that the ambient temperature changes by 300.degree. C., and this
value is about 1/50 of the conventional value.
As described above, the position detector of the present invention is of
the system which counts digitally the number of pulses, and determines the
position from the ratio t/T of the ultrasonic wave propagation time t
between the movable permanent magnet 5 and the fixed permanent magnet 5,
shown in FIG. 6(2), to the ultrasonic wave propagation time T between the
adjacent movable permanent magnets 14. Therefore, the detection error due
to the change of the ambient temperature around the waveguide is small,
and the position can be detected highly accurately.
* * * * *
|
|
 |
|
 |
Description |
|
